/Engine/Plugins/Runtime/GeometryProcessing/Source/DynamicMesh/Public/MeshSimplification.h
Does N rounds of collapsing edges longer than fMinEdgeLength. Does not use Quadrics or priority queue. This is a quick way to get triangle count down on huge meshes (eg like marching cubes output).
virtual void FastCollapsePass ( double MinEdgeLength, int Rounds, bool bMeshIsClosedHint, uint32 MinTriangleCount )